1. Field of the Invention
The present invention generally relates to a multichip-module fabrication method and a multichip module, and more particularly, to a multichip-module fabrication method and a multichip module in which a chip-mounting substrate is supported by a supporting base during a fabrication process of the multichip module.
Recently, to meet demands for higher-speed and higher-density in computers, a multichip-module is often utilized, in which semiconductor chips in the form of bear chips are directly mounted on the chip-mounting substrate.
Because mechanical strength of the chip-mounting substrate itself is weak, in the fabrication process of the multichip module, the chip-mounting substrate is mechanically reinforced by forming the supporting base in contact with a face of the chip-mounting substrate.
And the supporting base becomes unnecessary in a completed multichip module, so that the multichip-module fabrication process comprises a process which removes the supporting base from the chip-mounting substrate.
Therefore, in the removing process, in order to improve chip yield, it is important to carry out the removing process without damaging other components such as interconnection layers formed in the chip-mounting substrate.
2. Description of the Prior Art
FIGS. 1A, 1B show a process that removes a supporting base 2 from a chip-mounting substrate 1 (mother board) by means of a prior fabrication process of a multichip-module. As shown in FIG. 1A, the chip-mounting substrate 1 is constructed in the form that given-pattern interconnection layers 4 made of a conductive metal (ex. copper) are layered within an interlayer-insulating film 3 such as polyimide or BCB (benzocyclobutane). The supporting base 2 is made of, for example, silicon (Si), and is fixed to a bottom of the chip-mounting substrate 1.
As described above, the chip-mounting substrate 1 is constructed in the form that the interconnection layers 4 formed with the conductive films are layered within the interlayer-insulating film 3 which is mainly made with resinous materials, and thus the chip-mounting substrate 1 may be easily bent due to its weak mechanical structure. Then, when utilizing the chip-mounting substrate 1 without the supporting base 2, the chip-mounting substrate 1 may be bent during a fabrication process of the interlayer-insulating film 3 and the interconnection layers 4, and each of the layers will not be formed without damage.
Therefore, in the fabrication process of the multichip module, it is general practice that the chip-mounting substrate 1 is mechanically reinforced by forming the supporting base 2 in contact with the bottom face of the chip-mounting substrate 1.
And as the supporting base 2 is formed for the purpose of only reinforcement of the chip-mounting substrate 1, the supporting base 2 becomes unnecessary after completion of the multichip module. Thus, the multichip-module fabrication process comprises a supporting-base removing process that removes the supporting base 2 from the chip-mounting substrate 1.
In a conventional method, in order to remove the supporting base 2 from the chip-mounting substrate 2, the chip-mounting substrate 1 with the supporting base 2 is dipped in an etching pool filled with an etching medium. Subsequently, the supporting base 2 is removed from the chip-mounting substrate 1 by etching the supporting base 2. FIG. 1B shows the chip-mounting substrate 1 from which the supporting base 2 has been removed.
For ensuring support of the chip-mounting substrate 1 in each fabrication process of the multichip module, a thickness of about 650 .mu.m for the supporting base 2 is required. In order to entirely remove the supporting base 2, which has a relatively thick width, by means of etching, the chip-mounting substrate 1 with the supporting base 2 is generally needed to be dipped into the etching medium for about 24 hours.
The reason why it takes such a long time for the etching, is that the etching medium does not permeate a bonding section between the chip-mounting substrate 1 and the supporting base 2, and is also that the supporting base 2 is eroded from around the section (5 area) except this bonding section. Therefore, the etching process needs to be kept until the supporting base 2 is completely etched.
However, such a long time dip of the chip-mounting substrate 1 through the etching medium may cause that the etching medium to permeate among layers of the chip-mounting substrate 1 having a layer structure. Under this condition, as shown in FIG. 2, conductive-metal materials constructing the interconnection layers 4 may be disconnected with corrosion due to the permeated etching medium, and detachment in the interlayer-insulating film 3 may occur. As a result, a fabrication yield of the chip-mounting substrate 1 becomes low, and it causes a problem that fabrication efficiency and its stability are degraded.